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Sperm cells are so tiny that 400 to 500 million (400,000,000–500,000,000) of them fit onto 1 tsp.
Chronic marijuana use can damage the white blood cells and reduce the immune system's ability to respond to disease by as much as 40%. Without a strong immune system, the body is vulnerable to all kinds of degenerative and infectious diseases.
People who have myopia, or nearsightedness, are not able to see objects at a distance but only up close. It occurs when the cornea is either curved too steeply, the eye is too long, or both. This condition is progressive and worsens with time. More than 100 million people in the United States are nearsighted, but only 20% of those are born with the condition. Diet, eye exercise, drug therapy, and corrective lenses can all help manage nearsightedness.
In 2012, nearly 24 milliion Americans, aged 12 and older, had abused an illicit drug, according to the National Institute on Drug Abuse (NIDA).
Most childhood vaccines are 90–99% effective in preventing disease. Side effects are rarely serious.